1. Field of the Invention
The invention relates to orthopaedic wires and cables which have an improved resistance to failure over prior art wires and cables; and to methods for making such wires and cables.
2. Description of the Related Art
Multifilament cables and monofilament wires are used in a variety of orthopaedic applications. The cables and wires are typically made of stainless steel, titanium, or a cobalt-chromium alloy.
Multifilament cable generally provides superior fixation in trochanteric reattachment procedures. In addition, these cables perform exceptionally well, not only for trochanteric reattachment, but also for a wide variety of reconstructive and trauma applications. Many surgeons use cable for reduction and fixation of fractures of the femur, patella, humerus, and other bones. Cables are also used prophylactically for prevention of fractures in primary or revision total hip surgery.
In order for either wire or cable to perform well in clinical applications, it must possess certain properties. Cable manufacturers generally consider tensile strength to be the primary measure of wire and cable strength and the most important factor for clinical applications.
Wire is manufactured from forget billets using a rolling mill and a drawing bench. The preliminary treatment of the material to be manufactured into wire is done in the rolling mill where hot billets are rolled to round wire rod. The action of atmospheric oxygen causes a coating of mill scale to form on the hot surface of the rod and must be removed. This descaling can be done by various mechanical methods (e.g., shot-blasting) or by pickling, i.e., immersion of the wire rod in a bath of dilute sulphuric or hydrochloric acid. After pickling, the wire rod may additionally undergo a jolting treatment which dislodges the scale loosened by the acid. The remaining acid is removed by immersion of the wire rod in lime water.
The actual process of forming the wire is called drawing and is carried out on the metal in a cold state with a drawing bench. Prior art FIG. 1 shows a simple drawing bench 10. The wire 12 is pulled through a draw plate 14 which is provided with a number of holes, e.g. 16, (dies) of various diameters. These dies have holes which taper from the diameter of the wire 12 that enters the die to the smaller diameter of the wire 12' that emerges from the die. The thick wire rod 12 is coiled on a vertical spool 18 called a swift and is pulled through the die by a rotating drum 20 mounted on a vertical shaft 22 which is driven by bevel gearing 24. The drum can be disconnected from the drive by means of a clutch 26. To pass a wire through a die, the end of the wire is sharpened to a point and threaded through the die. It is seized by a gripping device and rapidly pulled through the die. This is assisted by lubrication of the wire. Each passage through a die reduces the diameter of the wire by a certain amount. By successively passing the wire through dies of smaller and smaller diameter, thinner and thinner wire is obtained. The dies used in the modern wire industry are precision-made tools, usually made of tungsten carbide for larger sizes or diamond for smaller sizes. The die design and fabrication is relatively complex and dies may be made of a variety of materials including single crystal natural or synthetic diamond, polycrystalline diamond or a mix of tungsten and cobalt powder mixed together and cold pressed into the carbide nib shape.
A cross section of die 16 is shown in prior art FIG. 2. Generally, the dies used for drawing wire have an outer steel casing 30 and an inner nib 32 which, as mentioned above, may be made of carbide or diamond or the like. The die has a large diameter entrance 34, known as the bell, which is shaped so that wire entering the die will draw lubricant with it. The shape of the bell causes the hydrostatic pressure to increase and promotes the flow of lubricant into the die. The region 36 of the die where the actual reduction in diameter occurs is called the approach angle. In the design of dies, the approach angle is an important parameter. The region 38 following the approach angle is called the bearing region. The bearing region does not cause diametric reduction, but does produce a frictional drag on the wire. The chief function of the bearing region 38 is to permit the conical approach surface 36 to be refinished (to remove surface damage due to die wear) without changing the die exit. The last region 40 of the die is called the back relief. The back relief allows the metal wire to expand slightly as the wire leaves the die. It also minimizes the possibility of abrasion taking place if the drawing stops or if the die is out of alignment with the path of the wire.
Although wire drawing appears to be a simple metalworking process, those skilled in the art will appreciate that many different parameters affect the physical quality of the drawn wire. Among these parameters, draw stress and flow stress play an important role. A discussion of the practical aspects wire drawing can be found in Wright, Roger N., "Mechanical Analysis and Die Design", Wire Journal, October 1979, the complete disclosure of which is hereby incorporated by reference herein.
Cable is manufactured by twisting multiple strands (filaments) of wire together. The material properties of cable are related to the material properties of the individual wire filaments which make up the cable. When considering the strength of multifilament cables, material strength expressed as stress only applies to the strength of individual cable filaments. Overall cable strength must be measured in terms of the actual load-carrying capacity of the cable. Cable strength is therefore be a function of two variables: the filament material strength and the cable construction. Improved cable strength can be achieved by increasing filament material strength, modifying cable construction, or both.
Other wire and cable properties include the following: ductility, a measure of the plastic deformation a cable or wire can withstand before fracturing; fatigue strength, a cable's ability to resist fracture under cyclic loading conditions; material toughness, the material's ability to absorb energy and resist fracture while under going plastic (non-reversible) deformation; ultimate tensile strength, the maximum stress achieved in a material before fracture occurs; and yield strength, the stress above which plastic (non-reversible) deformation of a material occurs.
As mentioned above, manufacturers of orthopaedic wires and cables generally consider ultimate tensile strength to be the most important property and have endeavored to provide cables and wires with very high tensile strength. Despite these efforts, clinical experience shows that even high tensile strength cables and wires can fail by fracture.